First
close-up view of Jupiter from Voyager 1. (Click on
the image for a larger view)

NASA
launched the two Voyager spacecraft to Jupiter, Saturn,
Uranus, and Neptune in the late summer of 1977. Voyager
1's closest approach to Jupiter occurred March 5, 1979.
Voyager 2's closest approach was July 9, 1979.

Photography
of Jupiter began in January 1979, when images of the brightly
banded planet already exceeded the best taken from Earth.
Voyager 1 completed its Jupiter encounter in early April,
after taking almost 19,000 pictures and many other scientific
measurements. Voyager 2 picked up the baton in late April
and its encounter continued into August. They took more
than 33,000 pictures of Jupiter and its five major satellites.

Although
astronomers had studied Jupiter from Earth for several centuries,
scientists were surprised by many of Voyager 1 and 2's findings.
They now understand that important physical, geological,
and atmospheric processes go on - in the planet, its satellites,
and magnetosphere - that were new to observers.

Discovery
of active volcanism on the satellite Io was probably the
greatest surprise. It was the first time active volcanoes
had been seen on another body in the solar system. It appears
that activity on Io affects the entire Jovian system. Io
appears to be the primary source of matter that pervades
the Jovian magnetosphere -- the region of space that surrounds
the planet, primarily influenced by the planet's strong
magnetic field. Sulfur, oxygen, and sodium, apparently erupted
by Io's volcanoes and sputtered off the surface by impact
of high-energy particles, were detected at the outer edge
of the magnetosphere.

This
time-lapse video records Voyager 1's approach to Jupiter
during a period of over 60 Jupiter days.

Particles
of the same material are present inside Io's orbit, where
they accelerate to more than 10 percent of the speed of
light. It is clear to scientists from a comparison of data
from Pioneers 10 and 11 (which flew past Jupiter in late
1973 and 1974) and the Voyagers that something changed in
the four and one-half years between the Pioneer and Voyager
encounters.

It
is not entirely clear just how far-reaching those changes
are, or what brought them about. They may be related to
Ionian activity. It is difficult to imagine, however, that
at least some of Io's volcanoes were not erupting when the
Pioneers flew past; it is also, the Voyager scientists say,
difficult to believe the Pioneers' instruments failed to
see magnetospheric concentrations of sulfur detected by
both Voyager spacecraft (Voyager 1 saw greater concentrations
than Voyager 2).